Iron is a metallic element that possesses strong supernatural properties, and is thus used by hunters when combating certain supernatural creatures.

Creatures that Iron effects

Demons- touching iron will burn a demon, they also can't cross iron, much like salt. Their weakness to iron is so strong, that even Azazel, one of (if not the) most powerful demons ever to exist, who was immune to many other demon weaknesses, was still affected by Iron (possibly because it was a Devil's trap). The only known demon to be immune to Iron is Samhain.

Ghosts- To be stabbed or shot by iron, will temporalily make the ghost disappear, they also can't cross iron, much like demons. Sam and Dean considered cold iron to be the best way to repel vengeful spirits.

Fairies- All fairies are weakened by iron, although powerful ones like Leprechauns can shrug it off.

Phoenix- Iron burned the Phoenix's skin, and it couldn't touch Iron, or if imprisoned by it, escape.

Leshi- The only way to kill a Leshi, is to cut of its head with an iron axe.

Shtriga-Iron is the only known thing that can kill a Shtriga

Iron also effects humans who enter full astra state, exactly the same as it does to ghosts, they can be imprisoned by it, to touch it causes them pain and makes them temporaily disappear, although like ghosts, they only reappear inside the iron prison.